An analog to digital converter generating a number of corresponding voltages in response to a number of values of a grey level is provided. The analog to digital converter includes a decoder and an operational amplifier. The decoder provides first to third output voltages having the same level when w most significant bits (MSBs) of the grey level correspond to the same value, provides first and second intermediate voltages in response to the x MSBs next to the w MSBs when the w MSBs correspond to different values, and selectively has the first to the third output voltages equal to one of the first and the second intermediate voltages. The operational amplifier obtains a pixel voltage by interpolating the first to the third output voltages, wherein the sum of w and x is smaller than or equal to the bit number of the gray level.